Inlining enabled by -mir-opt-level > 1 is incompatible with coverage

[rust.git] / compiler / rustc_session / src / config.rs

diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index c9ddcbdb5f5cc71bc4158e091b561ce90d867d69..e42889670b9dc1cc8e5e395aff2d9c7edfd06794 100644 (file)
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -1830,11 +1830,17 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
         }
 
         if debugging_opts.mir_opt_level > 1 {
+            // Functions inlined during MIR transform can, at best, make it impossible to
+            // effectively cover inlined functions, and, at worst, break coverage map generation
+            // during LLVM codegen. For example, function counter IDs are only unique within a
+            // function. Inlining after these counters are injected can produce duplicate counters,
+            // resulting in an invalid coverage map (and ICE); so this option combination is not
+            // allowed.
             early_warn(
                 error_format,
                 &format!(
-                    "`-Z mir-opt-level={}` (any level > 1) enables function inlining, which \
-                    limits the effectiveness of `-Z instrument-coverage`.",
+                    "`-Z mir-opt-level={}` (or any level > 1) enables function inlining, which \
+                    is incompatible with `-Z instrument-coverage`. Inlining will be disabled.",
                     debugging_opts.mir_opt_level,
                 ),
             );